minor in-code cleanup, documentation

minor in-code cleanup, documentation

i wanted to remove the 'only draw diff'-todo and looked into where i was
calling show_playfield(). i didn't call it anywhere where i expected it,
so i fired up gdb. gdb told me show_playfield() gets called from the
input loop after the switch(getchar()). i was puzzled about that,
because it fired even when no keys were pressed. then i noticed that
everytime the signal handler fires, a STX (ASCII 0x02) gets pushed onto
stdin.  WHO FIGURES?!  i was unable to find anything in the
documentation of sigaction or on the internet that indicates this!